Monitoring electrical signal parameters such as voltage, current, etc. and providing a permanent record thereof is either a necessity or highly desirable in countless technical fields. Frequently such monitoring must or should preferably be done in real time as the signal occurs. Electromechanical recording and displaying devices have been designed to fill this need at minimal expense.
Perhaps the least expensive and most readily commercially available of these devices is a strip chart recorder. A strip chart recorder is a device in which paper is moved past a printing mechanism, which also may move, for simultaneously recording one or more varying signals as a function of time. In many commonly available strip chart recorders the printing mechanism is at least one pen which is mechanically moved transversely across the paper as the paper is moved past the same.
Because of the inherently slow response time of such electromechanical mechanisms, strip chart recorders have a severely limited frequency response of from zero to at most approximately 150 Hz for nearly all commercially available units. Moreover, in order to obtain suitably legible waveforms at the higher frequencies within this limited frequency response range, the paper must move past the printing mechanism with ever increasing velocity, resulting in consumption of great quantities of paper and requiring more frequent replacement of the paper source.
One solution to the limited frequency response of inexpensive strip chart recorders has been to record higher frequency input signals with an analog FM recorder or to utilize a computer to digitize the input signal for subsequent storage. The recorded input signal may be reproduced at a slower rate within the response range of the strip chart recorder. Such solutions require further expensive equipment and prohibit the on-line recording of the input signals in real time.
Another approach is employed in sampling oscilloscope circuits manufactured by Tektronix, Inc. of Beaverton, Oregon and is discussed more fully in the book entitled Sampling Oscilloscope Circuits by John Mulvey and published by Tektronix, Inc. in 1970. This approach recognizes that, at least in so far as electrical signals originating from transducers for taking physical measurements is concerned, most such signals do not vary significantly in successive periods and that the occurrence or transient phenomena which appear for only a single cycle is rare. Moreover, where a transient event is of interest, such as in starting large motors, mechanical vibration analysis or temperature measurement, its duration frequently lasts for several seconds, over a large number of cycles, and any change normally occurs in the magnitude of the electrical signal, rather than that of the signal waveform.
Accordingly, the sampling oscilloscope approach recognized that a high frequency input signal may be reproduced at a lower frequency without significant loss of signal information by the repeated sampling of the high frequency input signal by an analog sampling circuit. By insuring that sampling occurs at a high repetition rate, an oscilloscope with a slowly responding trace may present the sampled values as a continuous waveform. However, the analog circuitry with which this "scaling down" or converting in frequency is achieved is both costly and complex. Moreover, as a result of the idiosyncracies of oscilloscopes, consecutive traces must begin their displays with the same sampling point, preventing sampling and resulting in loss of information for the period of time during which the oscilloscope returns to its initial sampling point.